Near field RF (radio frequency) communication requires an antenna of one near field RF communicator to be present within the alternating magnetic field (H field) generated by the antenna of another near field RF communicator by transmission of an RF signal (for example a 13.56 Mega Hertz signal) to enable the magnetic field (H field) of the RF signal to be inductively coupled between the communicators. The RF signal may be modulated to enable communication of control and/or other data. Ranges of up to a couple of centimeters are common for near field RF communicators.
Near field RF communication may be referred to as near-field communication (NFC). NFC communicators are capable of both initiating a near field communication (through transmission or generation of an alternating magnetic field) with another NFC communicator and of responding to initiation of near field communication by another NFC communicator. An NFC communicator may operate in a “reader” or “initiator” mode in which the NFC communicator seeks to initiate near field communication or in a “tag” or “target” mode in which the NFC communicator is receptive to initiation of near field RF communication. An initiator NFC communicator will generate an RF field and a target NFC communicator will respond by modulation of the received field, usually by load modulation. Examples of near field RF communicators are defined in various standards such as ISO/IEC 18092, ISO/IEC 14443, ISO/IEC 15693 ISO/IEC 21481, for example. Examples of NFC communicators can be found in ISO/IEC 18092 and ISO/IEC 21481 in particular.
NFC communicators may be provided as standalone or discrete devices or they may be incorporated within or coupled to larger electrical devices or host devices to enable those devices to communicate using NFC with other NFC communicators or devices incorporating or coupled to such near field RF communicators. When incorporated within a larger device or host, an NFC communicator may be a discrete entity or may be provided by functionality within the larger device or host. Examples of such larger or host devices are mobile telephones, portable computing devices (such as personal digital assistants, notebooks, laptops), other computing devices such as personal or desktop computers, computer peripherals such as printers, or other electrical devices such as portable audio and/or video players or other media players, CD players and DVD players. Other examples of such larger or host devices are consumer electronic products such as domestic appliances or personal care products, and other electrical or electronic devices, apparatus or systems. Some areas of application are payment systems, ticketing systems, for example in tickets (e.g., parking tickets, bus tickets, train tickets or entrance permits or tickets) or in ticket checking systems, toys, games, posters, packaging, advertising material, product inventory checking systems and so on.
NFC communicators need to be capable of both initiating an RF H field and of receiving an RF H field. Moreover a single NFC communicator design may be used for many different applications and in some of those applications the NFC communicator may function like an NFC reader and in other applications the NFC communicator may sometimes function like an NFC tag and in other cases it may function as both an NFC reader and an NFC tag, or peer to peer mode. The antenna circuit used in an NFC communicator can be designed to be optimized for use as a reader (to enable initiation or generation of an RF H field and to detect modulation or affects on that initiated RF H field) or as a tag (to receive an RF H field and to modulate that H field, for example by load modulation). However, due to conflicting requirements it is generally difficult to optimize it for both functions.
More particularly, when the communicator is used as an initiator, it is desirable to generate as much energy as possible at the transmitting frequency. In this way the distance at which the initiator can sense the tag can be maximized. This requires the NFC antenna to have very high quality factor (e.g., Q factor). However, if this same device is used as a tag, it needs to load modulate the signal. Load modulation (which is AM modulation in the case of NFC) creates two side bands around the carrier frequency. However, if the antenna element has a high quality factor, it will suppress the side-band signals. As a result, for a given antenna, the receiving (tag) performance is a trade off from the transmitting (reader) performance.
One important test of reader performance is the strength of the H field that is generated, whereas an important test of tag performance is how much the voltage of a reader is be pulled down (i.e., load modulated). Of course, one ultimate performance parameter is the maximum distance at which the reader or initiator can read a tag.